A variety of devices make use of magnetic sensors to orient themselves with respect to the Earth's Geographic Coordinates. More specifically, it is common to use a tri-axial magnetometer to measure the angle of the device with respect to the Earth's magnetic field, and a tri-axial accelerometer to measure the angle of the device with respect to the Earth's gravitational field. Together, these sensors can be used to estimate the device's pointing direction in magnetic bearing coordinates, unless the gravity and magnetic fields are either parallel or anti-parallel. Using a model of the Earth's magnetic field position, a correction estimate can be applied to re-reference the device from magnetic bearing coordinates to geographic coordinates, such as the Earth System of Coordinates.
For example, one such device comprises a borehole imager that is configured to obtain images of a borehole drilled into the Earth. Six sensors (a tri-axial magnetometer and a tri-axial accelerometer) attached to the device may be used to sense the imager's orientation as it moves within the borehole to record images, so that the resulting images of the borehole surface are aligned with geographic North. However, when imaging a borehole near formations that contain iron-oxides with magnetic fields sufficient to override the Earth's magnetic field as it appears at the magnetometers, the acquired sensor data at that location may contain magnetic field readings that are corrupt, or completely missing. A lack of orientation data may also be noticed whenever there is less than a full complement of sensors (e.g., less than the six sensors noted above) attached to the device.